Separation of trivalent 4f rare earths from molten fluoride compositions with uf3



United States Patent Oflice 3,379,648 Patented Apr. 23, 1968 ABSTRACT OF THE DISCLOSURE Trivalent 4f rare earth'fluorides are removed from molten fluoride by saturating the composition with UF and collecting the resultant precipitated phase.

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

Background of invention This invention relates to a method for removing rare earth fission products from molten fluoride reactor fuel systems or from molten fluoride solutions containing dissolved rare earth fluorides.

In molten fluoride reactor fuel system designed for thermal breeding, it is essential to maintain maximum neutron economy in order to obtain a net breeding gain. This implies that the accumulation of fisson products having a high capture cross section for thermal neutrons must be reduced to a minimum. In this respect, the rare earth elements (atomic numbers 57-71) particularly samarium, europium, neodymium, and gadolinium are generally considered tov account for more than 50% of the neutron absorption (i.e., poisoning), by non-gaseous elements. Neutron loss to these fission products must be kept low, and this should be done by, and it is an object of this invention to provide a method for, processing fissionable and fertile compositions on an economic, short batch, or preferably continuous cycle to remove rare earth fission product poisons from molten fluoride compositions.

Processes for the separation of rare earth fission product elements are known. They fall into three main classes. In one approach, a portion of the fluoride fuel is disengaged from a circulating fuel system, dissolved in aqueous acids, followed by solvent extraction purification. In another case, a portion of the fuel is fluorinated to form volatile fluorides. Vacuum distillation is employed to distill off fuel solvent fluorides such as HR BeF and ZrF dition has been shown to raise the liquidus temperature of the molten fluoride salt by at least 30 while removing appreciable quantities of uranium as well as plutonium. What is needed, and what this invention provides, is a scheme for the reprocessing of molten fluoride fuel mixtures which selectively and rapidly removes rare earth fission posions without any essential disruption in the composition of other solute or solvent components of the mixture.

Summary of invention The objects and advantages of the invention are realized from and based on the discovery that solid UF can be used as an eflicient and selective extractant for 4] trivalent fission product fluorides on an essentially continuous basis from molten fluoride fuel composition.

Description In practice, UP; is added to a molten fluoride composition containing rare earth fission products in an amount at least sufficient to saturate the melt with UF whereupon, a UF precipitate is formed which includes a portion of the race earth fission products from the melt. Precipitation may be effected at constant temperature or by cooling the melt after UF addition in order to decrease both rare earth fluoride and UF solubility.

This invention can be practiced as a short cycle batch process or in a continuous manner. In a short cycle sequence, the fluoride melt is saturated in UP cooled, and the resulting precipitate is collectedand separated from the melt. This sequence is repeated until the desired reduction in fission rare earths has been effected. To practice the invention as a continuous process, the circulating molten fluoride composition in a main stream is simply forced through a porous horizontal or vertical column packed with solid UF in a side stream until a desired rare earth decontamination factor is achieved.

This invention offers unique advantages as a short cycle or continuous process which removes high neutron capture cross section rare earth elements with essentially no contamination of the solution or disruption of the fuel system. The stability of UF and its ability to effectively remove rare earths without removing adverse amounts of tetravalent uranium (as UF from solution at tempera- Y tures up to about 875 C. is an especially significant ad- The non-distilled residue contains the fission product' poisons consisting principally of rare earths. Both of these processes are nonddeal because they involve disengagement of fuel, necessitating fuel make up. In addition, the

disengaged fuel stream is subject to several operations which, while standard, are nevertheless costly and require that the fuel and solvent fluorides be reconstituted before they can be returned to the reactor.

A third approach involves displacement or replacement of high cross section rare earth poisons by the addition of large quantities of a lower cross section rare earth fluoride such as cerium fluoride. However, the cerium advantage since it conserves the uranium fuel inventory.

In contrast to previous fuel reprocessing schemes, the solvent and solute compositions remain essentially constant except that rare earths have been removed and some slightly soluble UF has been added. If the amount of dissolved UF is regarded as excessive, it may be readily converted to UR, by oxidation, for example, by sparging with gaseous HF.

The effective use of UF as a solid extractant for trivalent rare earth fluorides is essentially independent of the solvent or solute components in the fluoride melt. The composition of each may vary over wide limits. All that is required is that the UF be present or be caused to crystallize or precipitate from solution. Nine typical fluoride compositions from which trivalent fluorides can be selectively removed by treatment with UF are listed in Table I below in which the numbers refer to concentration of the indicated metal fluoride in mole percent.

TABLE I.-COMPOSITION 4 The following examples will show the eflicacy of UP; In a separate experiment the solubility of La in a salt in removing selected trivalent rare earth fluorides (atomic of the same composition, without UF was tested. The numbers 57-71) from molten fluoride compositions. results are tabulated in Table -IV below.

TABLE IV Temp.(C.) 850 800 700 650 600 550 La solubility (moles) WithOUtUFa 0.0 .09 .09 .09 .064 0.05 La solubility with Us: .068 .052 .024 .013 .013

EXAMPLE I These results show that the decrease in the solubility of LaF as the combined function of temperature and a was added In m mcrementsfio pp UP addition is significantly greater than decrease in mately 2.2 kilograms of solution VII that initially consolubility achieved by temperature alone.

a tamed 10 mole fraction of CeF with about 1 micro EXAMPLE W curie of Ce 'as a radiotracer at a temperature of approximately 550 C. Filtered samples of the salt mix- A molten salt (solvent LiF=BeF '(66-34 mole perture were taken after each addition of UF and analyzed cent)) containing 0.18 mole/ kg. trivalent and 0.17 mole/ radiochemically for Ce. The results are summarized in kg. tetravalent uranium and trivalent samarium (.07 Table H below. mole/kg. of melt) was equilibrated for a period of time In general, rare earth removal may be related to U+ at 850 C. and then cooled to 550 C. After equilibration concentration in solution by the equation: at 550 C., the salt was filtered and the filtrate analyzed +3 for samarium. It was found that the samarium content lnNRe KlnN +constant had been reduced to approximately .03 mole/kg. of

where u+a are l fractions of rare earth and melt. In a parallel experiment under the same conditions tri n i respectively For the co-precipitation 'and concentrations except that the melt did not contain of CeF a value of about 0.55 has been obtained.. trivalent uranium, the final samarium concentration was 5 found to be unchanged. EXAMPLE H Having thus described my invention, I claim: All a lfimpefatum of C" a molten Salt composl' 1. A method for separating 4f rare earth trifiuorides tion consisting of 1.1 kg. of solution VII and containing f a molten fl rid composition which comprises y f Was mixed with P S of Solid a- After adding UF to said composition to form a saturated solueaCh addlllofl of [11:3, the molten S was filtered and 40 tion of UF precipitating UF from said solution, and the filtrate examine for its Nd content. The analytical separating the resulting precipitate.

results are given in Table III. 2. The method according to claim 1 in which the UF TABLE in UP; added (moles) 0 0.1 0.4 0.6 0.8 1 Nd in solution (mole fractlonXlO") 1 .93 68 .6 .31 0. 2

EXAMPLE HI F0 is added to the molten composition at a temperature 55 grams LaF were added to approximately 3 kilobelow 875 C. and precipitation of said UF is conducted grams of 66 mole percent LiF, 34 mole percent BeF at a temperature above the freezing point of the comand 6 mole percent Zrl i which also contained 3 mole position. percent UF The mixture was equilibrated at 850 C. 3. A method for separating 4f rare earth trifiuorides The temperature was reduced in C. intervals to 550 from a molten fluoride composition which comprises C. under equilibrium conditions. Filtered samples of the passing said composition through a porous column of salt mixture were withdrawn after each reduction in temsolid UF until a desired level of rare earth decontamiperature. nation has been etfected.

No references cited.

CARL D. QUARFORTH, Primary Examiner. L. DEWAYNE RUTLEDGE, Examiner.

M. I. SCOLNICK, Assistant Examiner. 

